With the development of modular multilevel converters, using the modular multilevel converter to construct DC grid has become a hot topic which attracts great concern in the power industry. Interconnection of DC power grids in different regions is an urgent problem that needs to be solved. Due to difference in functions of DC grids in different regions, that in technologies that are adopted, and that in constructing time, rated DC voltages of DC grids in different regions may be different. To interconnect DC grids with different rated DC voltages, DC-DC converters need to be used. In the power distribution level, a micro DC grid has become a hot topic, and interconnection of micro grids with different rated DC voltages requires DC-DC converters.
In the power transmission level, a traditional DC-DC converter usually employs a DC-AC-DC converting topology with two-stage AC/DC conversion (as discussed by Wang peibo, et al, Research on control of voltage stability in renewable energy system based on bi-directional high power DC/AC/DC converter, Journal of North China Electric Power University, vol. 36, no. 5, pp. 22-26, September 2009). A detailed scheme thereof can be summarized as: constructing two converters, DC terminals of the two converters are respectively connected to a first DC grid and a second DC grid (a rated DC voltage of the first DC grid is denoted herein to be lower than that of the second DC grid), AC terminals of the two converters are interconnected with each other via an AC circuit. A first converter conducts DC/AC conversion to convert DC power of the first DC grid to AC power, then a second converter conducts AC/DC conversion to rectify the AC power inverted by the first converter into DC power, which is then fed into the second DC grid.
A remarkable feature of the above-mentioned DC-DC converter is that there is no direct electrical connection between the DC terminals of the two converters, therefore, there is no direct electrical connection between the two DC grids, and the two DC grids are interconnected via the AC circuit. As such, firstly, since the rated DC voltage of the first converter and that of the second converter of the DC-DC converter are respectively equal to the rated DC voltage of the first DC grid and that of the second DC grid, it is impossible for the second converter to make full use of existing DC voltage of the first DC grid, and a high rated DC voltage of the second converter leads to increase in cost. Secondly, rated power of the first converter and the second converter of the above-mentioned DC-DC converter is respectively equal to transferred power between the first DC grid and the second DC grid, and overall power of the converter is 2 times the transferred power between the first DC grid and the second DC grid, which increases cost of the converter; thirdly, the transferred power of the convertor requires a two-stage full AC/DC (or DC/AC) conversion, which features great power loss; In addition, overall power rating of the AC circuit is two times the transferred power, capital investment in interconnection of AC transformers and that in phase reactors are high.